1. Field of the Invention
This invention relates to a camera and more particularly to an optical system driving device which is arranged to drive stepwise the photo-taking optical system of a camera.
2. Description of the Related Art
The conventional lens shifting mechanisms of the kind that draw out the lens of the camera stepwise by a reciprocating motion of a magnet have been arranged in varied types, including: a type which causes a moving coil to make a reciprocal motion by arranging the coil within a magnetic field obtained by a permanent magnet; and another type which causes the reciprocal motion of a moving yoke by means of a plunger type magnet.
The lens shifting mechanism of the kind using a magnet comprises a unidirectional feed mechanism which is driven by the magnet; a cam ring which has a cam face for drawing out the lens and driving teeth arranged to engage the feed pawl of the unidirectional feed mechanism and which is allowed to rotate only in one direction by a lock pawl arranged to engage the driving teeth; urging means which is arranged to urge rearward a lens carrying member carrying the photo-taking lens which is straightly shiftable in the direction of the optical axis thereof; and a screw which is provided on the lens carrying member and which is caused by the urging force of the urging means to be constantly abutting on the cam face of the cam ring. The cam ring is rotated to the degree of one tooth at a time by the feed pawl of the unidirectional feed mechanism in the cycle of "current supply to magnet"-"reverse current supply"-"stopping current supply" or "current supply"-"stopping current supply". The position of the lens is drawn out by the change in the height of the cam face of the cam ring which takes place in the optical axis direction accordingly as the cam ring is rotated. After the cam ring is thus caused to rotate to a degree corresponding to several teeth thereof, a photographing operation is performed by opening and closing a shutter while the cam ring is locked by its lock pawl with no current supplied to the magnet. In other words, each notch or graduation of lens shifting position corresponds to one of the teeth of the cam ring. Therefore, some length of time has been necessary for the cycle of current supply in drawing out the lens by one notch.
Compact cameras which are popular these days are using bifocal lenses or zoom lenses. Therefore, compared with the lenses of the past, lenses of long focal lengths are now in use. Compared with a lens having a short focal length, the lens of a long focal length has a narrower photographing (object) distance range within which the lens is focusable by shifting its position to another position. Therefore, for a given photographing distance range, the lens of a long focal length must be provided with a greater number of lens shifting notches or graduations than the lens of a short focal length.
Then, in order to shift the lens to a given position, the current supply cycle for the magnet must be repeated a number of times which is the same as the number of lens shifting graduations. Therefore, for the lens of a long focal length, the number of current supply cycles increases as much as the increase in the number of graduations. However, the reciprocating motion of the magnet involves the build-up time of the current supply, the delay of a mechanical action, etc.. Therefore, there is a limit to a possible reduction in time required for the reciprocating motion of the magnet.
Further, in turning the cam ring as much as one tooth, the tooth is moved by the feed pawl and is brought to a stop by means of the lock pawl. In actuality, however, in order to accurately stop the cam ring at every tooth, the cam ring must be shifted by the feed pawl to a certain degree beyond a one-tooth distance and then must be shifted back before it is brought to a stop by means of the lock pawl because of the dimensional precision of parts of the feed mechanism. Therefore, in shifting the cam ring to the degree of one tooth, a longer time is required than merely turning it by one tooth. The length of time required for shifting the lens further increases accordingly as the number of lens shifting graduations increases. Therefore, with the number of graduations increased by the increase in focal length, a shutter time lag which is required before a shutter actually opens after a shutter button is pushed also becomes longer.
Further, in respect of reduction in size of the camera, the size of the cam ring cannot be increased. It is, therefore, desirable to increase the number of the teeth by reducing the size of each tooth. However, there is a limit to the reduction in size of the teeth while ensuring reliable engagement of the feed claw and the lock claw. Hence, the possible number of teeth that can be formed on a cam ring of a given size is limited.
Therefore, it is difficult to increase the number of lens shifting graduations beyond a certain number.